Optical assembly for controlling beam size in bar code scanners

Abstract

In order to control the width of the scanning beam in the scanning direction over a significant range in front of the scanner, an optical assembly including a phase mask is utilized through which the beam passes after being configured into curved wavefronts, as by a lens which provides a focus in the vicinity of the far end of the range. The F/ number of the optical assembly is high and the cone of the beam in the scanning direction is small, for example, less than 5°. The mask is preferably transparent and has a region which may be rectangular, elliptical or circular forming a step through which the center of the beam passes which imposes a phase change with respect to the phase of the wavefronts which do not propagate via the region. The phase change may be uniform or in the form of a weak quadratic phase variation in this region of the mask which changes phase and thus slightly refocuses the beam. The transmissivity of the region may also be varied and reduced with respect to the transmissivity of the mask outside of the region. The beam may be scanned via a scanning mirror or by scanning the optical assembly.

What is claimed is: 
     
       1. An optical assembly for a bar code scanner comprising: a laser beam source which provides a laser beam which has a dimension in a scanning direction in which the laser beam scans across bars of a bar code;   said laser beam propagating outwardly from the scanner over a range having inner and outer ends closer and further, respectively, from the scanner, in which range the bar code can be located for scanning;   a lens for focusing said laser beam at a point located at the outer end of said range; and   an optical element for altering at least one of a phase or amplitude of a center portion of said laser beam, the optical element including, at most, two masking regions for performing the phase or amplitude altering by adjusting the width of the laser beam in the scanning direction so as to resolve bars not exceeding minimum width or being a minimum distance from one another.   
     
     
       2. The assembly of claim 1 wherein said optical element is operative to change the phase of the center portion of said laser beam. 
     
     
       3. The assembly of claim 2 wherein the width of said laser beam decreases between said inner and outer ends of said range. 
     
     
       4. The assembly of claim 1 wherein said optical element is operative to change the amplitude of the center portion of said laser beam. 
     
     
       5. The assembly of claim 1 wherein said optical element is operative to change both the amplitude and phase of the center portion of said laser beam. 
     
     
       6. The assembly of claim 1 wherein said optical element is operative to remove said center portion of said laser beam so as to limit the width of said laser beam in the scanning direction. 
     
     
       7. The assembly of claim 1 wherein said optical element includes a single masking region. 
     
     
       8. The assembly of claim 7 wherein the transmissivity of said optical element is different within said single masking region than without said single masking region. 
     
     
       9. The assembly of claim 7 wherein said masking region is rectangular. 
     
     
       10. The assembly of claim 7 wherein said masking region is radially symmetric. 
     
     
       11. The assembly of claim 1 wherein said optical element includes two masking regions. 
     
     
       12. The assembly of claim 11 wherein the transmissivity of said optical element is different within said two masking regions than without said two masking regions. 
     
     
       13. The assembly of claim 11 wherein said two masking regions intersect at the center of said optical element. 
     
     
       14. The assembly of claim 13 wherein each of said two masking regions is rectangular. 
     
     
       15. The assembly of claim 1 wherein said optical element further includes a quadratic phase variation element within said at most two masking regions. 
     
     
       16. The assembly of claim 15 wherein said quadratic phase variation element adds either a positive or negative wavefront curvature to said laser beam. 
     
     
       17. In a bar code scanner which provides a laser beam which has a width in a scanning direction which the laser beam scans across the bars of the bar code, a method for controlling the width of the laser beam, the method comprising the steps of: directing said laser beam outwardly from said scanner along an optical axis over a range having inner and outer ends closer and further, respectively, from the scanner, in which range the bar code can be located for scanning;   focusing said laser beam at a point located at the outer end of said range; and   adjusting the width of said laser beam by altering at least one of a phase or amplitude of a center portion of said laser beam;   said altering step comprising masking said center portion of said laser beam.